大小 (dàxiǎo) means 'size' and literally combines 大 (big) and 小 (small). It is used to describe or compare the dimensions or scale of objects. It often appears with 不同 (different) to express variation in size.
请选择合适的大小再下单。
Qǐng xuǎnzé héshì de dàxiǎo zài xiàdān.
Please choose the appropriate size before placing your order.
这两个房间的大小差不多。
Zhè liǎng gè fángjiān de dàxiǎo chàbuduō.
These two rooms are about the same size.
西瓜的大小不同,价格也不一样。
Xīguā de dàxiǎo bùtóng, jiàgé yě bù yīyàng.
Watermelons vary in size, and the prices are different too.